U.S. patent application number 10/685631 was filed with the patent office on 2009-05-07 for refractory metal core coatings.
Invention is credited to Daniel A. Bales, James T. Beals, Sudhangshu Bose, Glenn Cotnoir, Dinesh Gupta, John J. Marcin, Stephen D. Murray, Daniel Francis Paulonis, Joshua Persky, Keith Santeler, Venkat Seetharaman, Dilip M. Shah, Jacob Snyder, Carl Verner, John Wiedemer.
Application Number | 20090114797 10/685631 |
Document ID | / |
Family ID | 34377624 |
Filed Date | 2009-05-07 |
United States Patent
Application |
20090114797 |
Kind Code |
A1 |
Beals; James T. ; et
al. |
May 7, 2009 |
REFRACTORY METAL CORE COATINGS
Abstract
A refractory metal core for use in a casting system has a
coating for providing oxidation resistance during shell fire and
protection against reaction/dissolution during casting. In a first
embodiment, the coating includes at least one oxide and a silicon
containing material. In a second embodiment, the coating includes
an oxide selected from the group of calcia, magnesia, alumina,
zirconia, chromia, yttria, silica, hafnia, and mixtures thereof. In
a third embodiment, the coating includes a nitride selected from
the group of silicon nitride, sialon, titanium nitride, and
mixtures thereof. Other coating embodiments are described in the
disclosure.
Inventors: |
Beals; James T.; (West
Hartford, CT) ; Persky; Joshua; (Manchester, CT)
; Shah; Dilip M.; (Glastonbury, CT) ; Seetharaman;
Venkat; (Rocky Hill, CT) ; Bose; Sudhangshu;
(Manchester, CT) ; Snyder; Jacob; (Southington,
CT) ; Santeler; Keith; (Middletown, CT) ;
Verner; Carl; (Windsor, CT) ; Murray; Stephen D.;
(Marlborough, CT) ; Marcin; John J.; (Marlborough,
CT) ; Gupta; Dinesh; (South Windsor, CT) ;
Bales; Daniel A.; (Avon, CT) ; Paulonis; Daniel
Francis; (Higganum, CT) ; Cotnoir; Glenn;
(Thompson, CT) ; Wiedemer; John; (Glastonbury,
CT) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
900 CHAPEL STREET, SUITE 1201
NEW HAVEN
CT
06510
US
|
Family ID: |
34377624 |
Appl. No.: |
10/685631 |
Filed: |
October 15, 2003 |
Current U.S.
Class: |
249/175 |
Current CPC
Class: |
B22C 9/10 20130101 |
Class at
Publication: |
249/175 |
International
Class: |
B28B 7/28 20060101
B28B007/28 |
Claims
1. A casting system including a refractory metal core, said
refractory metal core having means for providing oxidation
resistance during shell fire and protection against
reaction/dissolution during casting, said oxidation resistance and
protection means being a coating consisting of aluminum
silicate.
2-3. (canceled)
4. A casting system including a refractory metal core, said
refractory metal core having means for providing oxidation
resistance during shell fire and protection against
reaction/dissolution during casting, said oxidation resistance and
protection means being a coating consisting of zirconium
silicate.
5. (canceled)
6. A casting system in accordance with claim 1, wherein said core
is formed from a material selected from the group consisting of
tantalum, niobium, tungsten, alloys thereof, and intermetallic
compounds thereof.
7. A casting system in accordance with claim 1, wherein said core
is formed from molybdenum.
8. A casting system including a refractory metal core, said
refractory metal core having means for providing oxidation
resistance during shell fire and protection against reaction
dissolution during casting, said oxidation resistance and
protection means being a coating consisting of an oxide selected
from the group consisting of calcia, and magnesia.
9-10. (canceled)
11. A casting system including a refractory metal core, said
refractory metal core having means for providing oxidation
resistance during shell fire and protection against
reaction/dissolution during casting, said oxidation resistance and
protection means comprising a coating contacting said refractory
metal core, said coating comprising a nitride selected from the
group consisting of silicon nitride, sialon, and mixtures
thereof.
12. A casting system including a refractory metal core, said
refractory metal core having means for providing oxidation
resistance during shell fire and protection against
reaction/dissolution during casting, said oxidation resistance and
protection means comprising a coating adjacent said core, said
coating comprising a carbide selected from the group consisting of
silicon carbide, titanium carbide, and mixtures thereof.
13. (canceled)
14-15. (canceled)
16. A casting system including a refractory metal core, said
refractory metal core having means for providing oxidation
resistance during shell fire and protection against
reaction/dissolution during casting, said oxidation resistance and
protection means comprising a coating, said coating comprising a
ceramic coating and a layer between the refractory metal forming
the refractory metal core and said ceramic coating, wherein said
layer is formed from a material selected from the group consisting
of TiC.sub.T and Si.sub.3N.sub.4.
17-25. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to coatings to be applied to
refractory metal cores to protect the cores from oxidizing during
shellfire and from reaction/dissolution during the casting
process.
[0002] Investment casting is a commonly used technique for forming
metallic components having complex geometries, especially hollow
components, and is used in the fabrication of superalloy gas
turbine engine components. The present invention will be described
in respect to the production of superalloy castings, however it
will be understood that the invention is not so limited.
[0003] Cores used in investment casting techniques are fabricated
from ceramic materials which are fragile, especially the advanced
cores used to fabricate small intricate cooling passages in
advanced gas turbine engine hardware. These ceramic cores are prone
to warpage and fracture during fabrication and during casting.
[0004] Conventional ceramic cores are produced by a molding process
using a ceramic slurry and a shaped die. The pattern material is
most commonly wax although plastics and organic compounds, such as
urea, have also been employed. The shell mold is formed using a
colloidal silica binder to bind together ceramic particles which
may be alumina, silica, zirconia, and aluminum silicates.
[0005] The investment casting process used to produce a turbine
blade, using a ceramic core is as follows. A ceramic core having
the geometry desired for the internal cooling passages is placed in
a metal die whose walls surround but are generally spaced away from
the core. The die is filled with a disposable pattern material such
as wax. The die is removed leaving the ceramic core embedded in a
wax pattern. The outer shell mold is then formed about the wax
pattern by dipping the pattern in a ceramic slurry and then
applying larger, dry ceramic particles to the slurry. This process
is termed stuccoing. The stuccoed wax pattern, containing the core
is then dried and the stuccoing process repeated to provide the
desired shell mold wall thickness. At this point, the mold is
thoroughly dried to obtain green strength and the wax removed by
application of high pressure steam which removes much of the wax
from inside of the ceramic shell. The mold is then fired at high
temperature to remove the remainder of the residual wax and to
strengthen the ceramic material for the casting operation.
[0006] The result is a ceramic mold containing a ceramic core which
in combination define a mold cavity. It will be understood that the
exterior of the core defines the passageway to be formed in the
casting and the interior of the shell mold defines the external
dimensions of the superalloy casting to be made. The core and shell
may also define other features such as core supports to stabilize
the core or other gating which acts to channel metal into the cast
component. Some of these features may not be a part of the finished
cast part but are necessary for obtaining a good casting.
[0007] After removal of the wax, molten superalloy material is
poured into the cavity defined by the shell mold and core assembly
and solidified. The mold and core are then removed from the
superalloy casting by a combination of mechanical and chemical
means.
[0008] Attempts have been made to provide cores for investment
casting which have improved mechanical properties, thinner
thicknesses, improved resistance to thermal shock, and new
geometries and features. One such attempt is shown in published
U.S. Patent Application No. 2003/0075300, which is incorporated by
reference herein. These efforts have been to provide ceramic cores
with embedded refractory metal elements.
[0009] While it has been recognized that coatings are desirable to
improve the performance of the refractory metal cores, there
remains a need to define particularly useful coatings. Currently,
chemical vapor deposition of aluminum oxide (alumina) is the
baseline process/composition primarily due to availability and the
excellent compatibility of alumina with molten nickel superalloys.
A significant coefficient of thermal expansion (CTE) mismatch
exists between the refractory metal/alumina that produces a
microcracked coating. In its microcracked condition, the baseline
coating is not entirely oxidation resistant during the investment
shellfire.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to provide coatings
for refractory core elements which have a reduced tendency for
microcracking.
[0011] It is a further object of the present invention to provide
coatings for refractory core elements which have improved oxidation
resistance.
[0012] The foregoing objects are attained by the coatings of the
present invention.
[0013] In a first embodiment, a refractory metal core for use in a
casting system has a coating for providing oxidation resistance
during shell fire and protection against reaction/dissolution
during casting. The coating comprises at least one oxide and/or a
silicon containing material or a stable oxide former.
[0014] In a second embodiment, a refractory metal core for use in a
casting system has a coating for providing oxidation resistance
during shell fire and protection against reaction/dissolution
during casting. The coating comprises an oxide selected from the
group consisting of magnesia, alumina, calcia, zirconia, chromia,
yttria, silica, hafnia, and mixtures thereof.
[0015] In a third embodiment, a refractory metal core for use in a
casting system is provided, which refractory metal core has a
coating for providing oxidation resistance during shell fire and
protection against reaction/dissolution during casting. The coating
comprises a nitride selected from the group consisting of silicon
nitride, sialon, titanium nitride, and mixtures thereof.
[0016] In a fourth embodiment, a refractory metal core for use in a
casting system is provided, which refractory metal core has a
coating for providing oxidation resistance during shell fire and
protection against reaction/dissolution during casting. The coating
comprises a carbide selected from the group consisting of silicon
carbide, titanium carbide, tantalum carbide and mixtures
thereof.
[0017] In a fifth embodiment, a refractory metal core for use in a
casting system is provided, which refractory metal core has a
coating for providing oxidation resistance during shell fire and
protection against reaction/dissolution during casting. The coating
comprises a ceramic coating and at least one layer between the
refractory metal forming the refractory metal core and said ceramic
coating.
[0018] In a sixth embodiment, a refractory metal core for use in a
casting system is provided, which refractory metal core has a
coating for providing oxidation resistance during shell fire and
protection against reaction/dissolution during casting. The
refractory metal core is formed from molybdenum and has an etched
surface. The etched surface may be formed using any suitable
technique known in the art. The coating comprises alumina which has
been chemically vapor deposited.
[0019] In a seventh embodiment, a refractory metal core for use in
a casting system is provided, which refractory metal core has a
base coating for providing oxidation resistance during shell fire
and protection against reaction/dissolution during casting, and
further has a top coat overlaying the base coating.
[0020] In an eighth embodiment, a refractory metal core for use in
a casting system is provided, which refractory metal core has a
coating for providing oxidation resistance during shell fire and
protection against reaction/dissolution during casting. The coating
comprises alternating layers of alumina and a material selected
from the group consisting of TiC, TiN, TiCN, and zirconia.
[0021] Other details of the refractory metal core coatings, as well
as other objects and advantages attendant thereto, are set forth in
the following detailed description.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0022] Refractory metal cores are a ductile based coring system for
creating intricate cooling channels in cast components. The
intricate metal cores are formed from refractory metals selected
from the group consisting of molybdenum, tantalum, niobium,
tungsten, alloys thereof, and intermetallic compounds thereof. A
preferred material for the refractory metal core is molybdenum and
its alloys.
[0023] One of the key components to high yield of the refractory
metal cores is a robust oxidation, dissolution/reaction barrier
coating applied to the refractory metal core. The coating protects
the refractory metal from oxidizing during shellfire and from
reaction/dissolution during the casting process. Depending on the
alloy (usually nickel based superalloys) and condition (equiaxed,
DS, SX), molten metal may be in contact with the refractory metal
core for a significant amount of time (SX) or be rapid (equiaxed).
The type/properties of coatings may vary for the different
conditions (i.e., SX castings require a much more effective
refractory metal core dissolution barrier than equiaxed).
[0024] The choice of the coating composition to be used and
application method is predicated by many factors. Chemical
compatibility with both refractory metal and cast alloy at process
conditions is one such factor. For example, while some reaction
with the refractory metal may be desired for good adherence,
extensive reaction may embrittle or limit leachability. Also,
active alloy additions require a more inert coating.
[0025] Another factor is physical property match. For example, a
coating which has a coefficient of thermal expansion (CTE) close to
that of the refractory metal is desirable to reduce mismatch
cracking during processing. Strain compliance or porosity of the
coating is another physical property which may be considered.
[0026] Yet another factor is the need for a thin and uniform
coating process to retain cast features, which favors
non-line-of-sight processes. With regard to leachability, it is
desirable that the coating be removable from casting without base
metal damage.
[0027] One useful coating to be applied to the refractory metal
core is a mixed oxide--alumina silicate composition wherein the
aluminum silicate may be mullite. Such a coating is advantageous
because it better matches the CTE of refractory metals. The coating
may include a silicon rich layer closer to the substrate for better
adherence and an alumina rich exterior for better compatibility
with active alloy additions. Zirconium silicate (zircon) is another
mixed oxide that may be used. It has a compatible CTE. The mixed
oxide coatings may be applied using a wide variety of application
methods including, but not limited to, chemical vapor deposition,
electrophoretic process, plasma spray techniques, etc.
[0028] Another useful coating include ceramic coatings formed from
oxides such as zirconia, yttria, hafnia, and mixtures thereof.
Alternatively, the coatings may include nitrides such as silicon
nitrides, sialon, titanium nitride, and mixtures thereof. Still
further, the coatings may include carbides such as silicon carbide,
titanium carbide, tantalum carbide, and mixtures thereof. The
coating may also be a silicide such as molybdenum disilicide.
[0029] One technique which may be used to improve the coating
applied to the refractory metal core involves vapor honing/acid
etching and anodic etching to increase mechanical bonding of CVD
deposited alumina on molybdenum.
[0030] One or more interlayers can be used to help increase
adherence of a ceramic coating as well as increase oxidation
resistance. The layer or layers between the refractory metal, such
as molybdenum, and the ceramic can be applied by plating or other
coating means. The layer(s) may be formed from a metal selected
from the group including nickel, platinum, chromium, silicon,
alloys thereof, and mixtures thereof. Alternatively, the layer(s)
may be formed from intermetallics such as NiAl, MCrAlY, MoSi.sub.2.
Carbides and nitrides, such as TiC, TiN, and Si.sub.3N.sub.4, may
be used between a refractory metal/oxide coating or directly
between a molybdenum/oxide.
[0031] In yet another embodiment of the coatings of the present
invention, the oxidation resistance of the refractory metal core
can be increased by over coating the base coating. The over coating
may be a ceramic, such as multi-layered alumina, chromia, yttria,
and mixtures thereof; metals, such as nickel, chromium, platinum,
alloys and mixtures thereof; and/or intermetallics, such as
aluminides, silicides, and mixtures thereof. Over coats can be
applied by plating, chemical vapor deposition, or other coating
methods.
[0032] In still another embodiment, the coatings of the present
invention may include laminate coatings. In these coatings,
multiple alternating layers of coatings may be used to help
increase adherence, reduce CTE mismatch, and/or nucleate a more
uniform structure. Examples include TiC, TiN, TiCN/alumina and
zirconia/alumina.
[0033] In yet another embodiment, the coatings of the present
invention may be thermally grown coatings applied for oxidation
resistance to form a dissolution barrier during shell fire.
Examples include chromium plate to chromia, aluminide to alumina,
and silicide to silica.
[0034] A number of different processes may be used to apply the
coatings of the present invention to the refractory metal cores.
These processes include electrophoretic (EPD) process, i.e., an
electrochemical method of depositing powder based coating that can
be ceramic, metal, or intermetallic. This is a non line of sight
process that offers flexibility in chemistry, structure, and
layers. An EPD process can also be aqueous based and low cost.
[0035] Another process is dip coating techniques using a sol-gel or
preferably a high solids yield coating to create a film. Dip
coating reduces line of sight issues.
[0036] Physical vapor deposition methods may be used. These methods
include a wide array of coating processes including EB-PVD,
cathodic arc, plasma spray, and sputtering.
[0037] Diffusion coating techniques may also be used. Diffusion
coating includes processes such as aluminiding, siliciding,
chromizing, and combinations thereof. Oxygen active elements, such
as yttrium, zirconium, hafnium, etc., and noble metals such as
platinum may be incorporated to form better lasting oxide scales.
The coating process may be followed by controlled oxidation to form
oxide scales.
[0038] An oxide coating may be formed on the refractory metal cores
during the preheating of a DS/SX mold in an air furnace up to
1000.degree. C. before putting it into a vacuum furnace to shorten
the heat up cycle.
[0039] It is apparent that there has been provided in accordance
with the present invention refractory metal core coatings which
fully satisfy the objects, means, and advantages set forth
hereinbefore. While the present invention has been described in the
context of specific embodiments thereof, other alternatives,
modifications, and variations will become apparent to those skilled
in the art having read the foregoing description. Accordingly, it
is intended to embrace those alternatives, modifications, and
variations as fall within the broad scope of the appended
claims.
* * * * *